I have the a5+...love them! but searching for a more low-end, at lowered volume. Since with a new baby I cannot crank it as often as I would like. So I thought EP125 would fill that space nicely. Is that a good choice?

Also looked at the Audioengine S8 but I would pick Axiom since both are about the same price.

I've no experience with EP125, but my guess is it's only for the smallest of rooms. I had an EP175 in my living room of 18 X 12 X 10. It would have been fine at lower volumes, but not at the volume I crank it to, so I upgraded to the EP350, and now thinking about adding a second sub.

You can always try it and if you don't like it, send it back for a refund of all but return shipping, unless it's for an upgraded product.

Looking on the Products page for this Subwoofer, and looking at the amp picture, it has high-level inputs, so yes, you can attach amplified speaker wires to this subwoofer.

I think you would use the line-out of the A5 to connect to the sub. Assuming that line-out has variable volume (which it may or may not), the volume control would be from the speaker control. You'd probably be ok with an EP125 in that room, but there are very few people around here who have an EP125 or have even heard one, sorry to say.